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Topic: Engine/tuning issues (Read 2621 times)

If it dies when you open the throttle quickly, that is indicative of it being a big, long stroke single. At low engine speeds a quick blip of the throttle will drastically change engine vacuum, but there isn't enough air velocity through the carb for the carb to work. Gas won't be picked up through the jets so it runs lean and stalls. Learning to roll the throttle on will eliminate most of this. This feature is why car carbs had an accelerator pump to pump in more fuel at low engine speed/large throttle openings. You can tune some of it out, but eliminating it requires an accelerator pump or jetting it so rich that you almost instantly foul plugs. When properly jetted you will never notice this issue with proper throttle technique.

Good to know that's normal. Would a larger carb (like a vm32) overcome this at least somewhat?

I'm more familiar with car carbs, which like you say have accelerator pumps among other differences, so I was a bit baffled by this little vm28.

How do I more precisely set the timing? I'm lost without timing marks.

Most people use "ping timing" to see how far it can be advanced before "pinging" occurs. The hemi has a big combustion chamber, so it likes plenty of spark advance to help it have time to burn the fuel in such a large area.

To "ping time", you bring the bike to a fairly substantial long hill, and start riding up it and get into top gear at around 2800-3000 rpm. If you don't have a tach, but have standard final drive sprocket, it's around 45-48mph in top gear.If the bike doesn't "ping", then you can advance it a little bit. You can keep doing this until you start hearing some "ping". That's as far as you can advance it safely. It will only "ping" under worst case conditions, which are to be avoided if possible, since you don't ever really want to have any "ping". This is just telling you how far you can advance until you get it. Ping is detonation, so you don't want to be having it, but you can use it to determine your limits. Then you can back it off a hair in the retarded direction to keep it safely out of "ping" territory.

If you don't know what ping sounds like, it's that metallic rattling noise that you hear when you try to go up a hill in top gear at a very low rpm, and the engine starts making that noise. Or if you got low octane fuel by mistake, and the engine is pinging. That's the noise.Some people call it "knock" or "engine knocking". Same thing, different name.

CMW should have them in stock. It should not take weeks to get. That being said, most of them are not "perfectly" straight. Sometimes the adjusters are slightly off. This may be an issue if you miss a shift and over rev the heck out of it-it would be somewhat more like to spit a push rod. In regular use you will never notice it. This will NOT cause a rough idle condition. Improper valve adjustment-too tight especially- can cause rough running. So being too bent to properly adjust will affect running. But these bikes are actually pretty forgiving of small valve mis-adjustments. So if they are BENT, that is an issue. If there is a slight mis-alignment that you have to search hard to find, then probably not.

There you go. Call your nearest dealer or CMW. You'll have a set in a few days. In the mean time try adjusting it at the tightest spot. Just for testing purposes, not for normal riding. This will insure that the valve fully closes. Does that help your idle? If so, you found the problem.

You haven't mentioned if you made other changes. Do you have the stock air box and filter? Did you remove the crank case vapor catch can? Have you drained it? Is the crank case vent hose clogged? Have you changed gas stations lately? Was it left out in the rain? How old is the battery? Is it charging correctly? How is the connection and resistance for the plug cap? Do you have an Autolite (NO!!) plug or an NGK? Did the issue start suddenly or has it been building for a while? Have you checked for air leaks in the intake hose and manifold?

Thanks for the input, Vince. This thing has me demoralized at the moment. It does run, just not like I feel a rebuilt engine should.

This is after a top end rebuild. 535cc alloy cyl, 2mm spacer, forged piston, valve job. Stock air box & filter as far as I can tell. I cleaned the filter. Catch can is there. I have drained it but not recently -- I'll do that just in case. Hose is clear, brand new actually. Fresh gas. Battery is new. Can't find any air leaks. Plug is NGK, but I haven't checked the resistance -- will do that. Gap is slightly larger than it should be (~.02") I think I'll throw in a new one and a bosch coil just for the heck of it. I'll see how it runs once those arrive in the mail... My local dealer is less than helpful when it comes to parts for the iron barrels.

I took it to an engine specialist today. He said he wouldn't worry about it until the rings are seated. He also said I should really split the case and check out the lower section soon, he thinks something is amiss down there. I was hoping I could wait on that... But my bike does shake like it's trying to erase an etch-a-sketch.

Yes, it will take quite a number of miles before those rings are fully bedded-in.The basic sealing gets done early, but the rest of it takes quite a few miles.Idling won't do it. You need to load the rings up with some acceleration and deceleration with some real road riding. You don't have to rev it way up or be hard on it. Just give it some loading on some hills and stuff, at moderate speeds.That will help. You are probably running on lower compression than you expected, until the rings seat better.

If it's shaking like a paint-shaker downstairs, that's not good.It usually is a sign that the crank was not trued well at the factory. The ones that are trued better don't shake as much.These bikes are not inherent shakers, just because they are singles. They can be a lot smoother than many people think, if the crank just gets trued properly.

FRESH REBUILD!!!!! These engines take a long time to break in. City driving is excellent for this. It will run progressively better with time and miles. HOWEVER... This affects some of the previous answers. You might benefit from a new carb. Carbs operate on the principle of volumetric efficiency. That is- they are efficient within a range of air volume and velocity. A larger piston changes both velocity and volume. You may need one larger or even smaller that is compatible with the changes you made. You may also need a new air intake system to accommodate the new volume potential. That larger piston also affects the combustion process. The ignition flame front may not reach the edge of the piston. You may have incomplete combustion or prematurely complete combustion. This may require reshaping the chamber, or perhaps larger valves, or both. Then you have to address the exhaust system. It needs to be compatible with the changes in flow. An engine with valve overlap needs an exhaust system that will scavenge the combustion chamber, then create a reverse wave to block the open exhaust valve to keep the incoming charge in the chamber rather than dumping out the still open exhaust valve. Volumetric efficiency applies to exhaust systems also. So you may never get the smooth idle you want. Or it just might need to break in. As long as it is not running lean to cause engine damage, you might just try running it for a while. Or... I have concerns about you bottom end. This should have been addressed before the over bore. I have seen many bikes blow out the crank with a new top end. The tired bottom end could not take the extra power. A marginal main bearing will cause the crank end to oscillate- rough running condition. A bad rod bearing could allow the piston a little farther into the combustion chamber at TDC. This could affect flame front consistency. I'm talking .001" imperfections. At this point it needs a professional evaluation. He said to wait and see. I would say the same, but I will add that you should look over the points I raised above and formulate a plan to at least investigate these points when the break in is much further along.